1. Field of the Invention
The present invention generally relates to a focus control apparatus of a player for playing a multilayer recording disc and, more particularly, to a focus control apparatus for controlling an optimum light converging position of a read light which is irradiated onto a predetermined recording surface of a disc for the recording surface.
2. Description of Related Art
Nowadays, an information recording medium of a high recording density and a large capacity called a digital video disc or a digital versatile disc (abbreviated as "DVD") and a system utilizing the medium are to be widely used. The DVD disc has a double-layer recording type in which information recording surfaces are formed on upper and lower two layers sandwiching an intermediate layer (spacer or space region). To read data by an optical pickup from one of the disc surface sides of the disc of such a type, a focal point (focal position or optimum light converging position) of read light has to be positioned on the information recording surface of a desired one of the layers.
Usually, a focus jumping operation for moving a focal point of read light from an information recording surface of one of the layers to an information recording surface of the other layer is executed based on a zero-cross point detection of a focusing error signal which is generated based on a photo-sensing output of a pickup.
Specifically, the pickup has a focusing actuator for displacing the focal point of read light in the direction perpendicular to the disc surface by displacing an objective lens which is, for example, a read light radiating optical system in the optical axial direction and determines the focal point. An acceleration signal as a focus jump activation signal for moving the focal point of the read light to a target recording surface is supplied to the focusing actuator in the initial stage of the focus jumping operation. On the basis of the timing of a zero-cross point which is sequentially detected from the focusing error signal obtained during the displacement of the focusing actuator in response to the acceleration signal, a series of operations for supplying a deceleration signal for finishing the supply of the acceleration signal or stopping the displacement of the focusing actuator in response to the acceleration signal to the focusing actuator and, further, for finishing the supply of the deceleration signal and restarting a focusing servo to the target recording surface is executed.
FIG. 1 shows a relationship between a position in a disc internal layer of a focal point P of read light which is determined by an objective lens 100 and a level FE of a focusing error signal obtained when the focal point P is moved in the optical axial direction in the case where a double-layer recording disc is read. It will be understood that the focusing error signal basically shows the zero level (zero-cross points ZC00 and ZC10) in a state where the focal point P is on the recording surface and draws an S-shaped curve around the zero level as a center. A period from the minimum value to the maximum value of one S-shaped curve generally corresponds to a control range of a focusing servo loop which is formed. Since the supply control of the acceleration and deceleration signals is executed during the focal point movement between one recording surface and the other recording surface, it is performed on the basis of detection timings of zero-cross points ZC00, ZC01, ZC1-1, and ZC10 of the focusing error signal as shown in the diagram.
Recording marks such as pits for showing an information signal are formed in a line like tracks on the recording surface. It is necessary to make the focal point of the read light trace the tracks in order to sequentially and correctly read the recording marks. For this purpose, a tracking servo is applied. For a disc such as the conventional compact disc (CD) in which an information recording surface is formed only on a single layer, a series of set-up operations is preformed in a manner such that an operation for allowing the focal point of the read light to approach the recording surface from a far position from a state where the focusing servo and the tracking servo are turned off, the focusing servo is applied in response to the arrival of the focal point at the recording position on the basis of the S-shaped curve of the focusing error signal, and after that, the tracking servo is exercised. This set-up operation is an operation for reading the disc again just after turn-on of a power source of a player, just after the loading of the disc into the player, or the like.
When the set-up operation which is a standard operation in the CD, however, is applied to the focus jumping operation of the DVD of the double-layer recording type, problems occur.
That is, the tracking error signal exerts an influence on the focusing error signal due to a crosstalk caused by a construction in which the focusing system and the tracking system are optically, electrically, and mechanically close to each other in the pickup. Especially, the tracking error signal when the tracking servo is off fluctuates more largely than a case where the tracking servo is on. Consequently, in a conventional focus jump executed with the tracking servo being turned off, there can be a case that the generated focusing error signal does not draw an ideal S-shaped curve as shown in FIG. 1.
In this case, due to the occurrence of a pseudo zero-cross point different from a desired zero-cross point, the supply control of the acceleration and deceleration signals cannot be correctly executed. Accordingly, the focusing actuator can be erroneously operated and it leads to a failure of the focus jumping operation.